I suspect that the coupler is for link and pin, but has two different height slots, which makes it look more like a large coupler. Here is a shot of it in passenger service.
Here are 3 early shots of SVRy passenger car end platforms. Note the use of link and pin in 1909. The second shot is dated 1912 with an automatic coupler and I have no idea on the date of the 3rd shot. They all have one thing in common which is that the end platforms were originally set up for the Miller Hook. Notice in the 1909 shot the square hole where the Miller buffer would have been. In the two additional shots the Miller buffer is still in place.
Here is a narrow gauge Miller hook. Our car shop seems to have become the home of both a rat(s) and an owl over the winter, and has not been cleaned up due to the roof damage mentioned in another post. This Miller hook if I got the story right was dug out some years back from the river bank down in S. Baker where other discarded parts had been dumped in the early days. It would be interesting to know if SVRy actually used the Miller hook at some point, or if conversion to link and pin was done when the used pass. cars arrived from U&N and OSL.